A plate heat exchanger comprises a plate pack of a number of assembled heat transfer plates forming between them plate interspaces. In most cases, every second plate interspace communicates with a first inlet channel and a first outlet channel, each plate interspace being adapted to define a flow area and to conduct a flow of a first fluid between said inlet and outlet channels. Correspondingly, the other plate interspaces communicate with a second inlet and a second outlet channel for a flow of a second fluid. Thus the plates are in contact with one fluid through one of their side surfaces and with the other fluid through the other side surface, which allows a considerable heat exchange between the two fluids.
Modern plate heat exchangers have heat transfer plates, which in most cases are made of sheet metal blanks which have been pressed and punched to obtain their final shape. Each heat transfer plate is usually provided with four or more “ports” consisting of through holes punched at the four corners of the plate. In some cases, additional ports are punched along the short sides of the plates so as to be located between the ports punched in the corners. The ports of the different plates define said inlet and outlet channels, which extend through the plate heat exchanger transversely of the plane of the plates. Gaskets or some other type of sealing means are alternatingly arranged round some of the ports in every second plate interspace and, in the other plate interspaces, round the other ports so as to form the two separate channels for the first and the second fluid, respectively.
Since considerable fluid pressure levels are obtained in the heat exchanger during operation, the plates need to be sufficiently rigid so as not to be deformed by the fluid pressure. The use of plates made of sheet metal blanks is possible only if the plates are somehow supported. This is usually achieved by the heat transfer plates being formed with some kind of corrugation so that they bear against each other at a large number of points.
The plates are clamped together between two flexurally rigid end plates (or frame plates) in a “frame” and thus form rigid units with flow channels in every plate interspace. The end plates are clamped against each other by means of a number of clamp bolts which engage both plates in holes formed along the circumference of each end plate. In some plate heat exchangers, the plates are joined by welding or soldering, in which case the purpose of the end plates is to protect the heat transfer plates of the heat exchanger.
When designing a plate exchanger of the above type for use at relatively high pressures, special considerations have to be made. A heat transfer plate which is intended for use in applications involving relatively low pressures may have a large heat transfer surface. If said fluid is supplied under high pressures, the large heat transfer surface will cause great forces which must then be absorbed by the frame or the solder between the plates.
The bending moment exerted on an end plate owing to the liquid pressure is proportional to the width of the plate raised to the second power. At pressures of 100-150 bar (10-15 MPa) extremely thick end plates are necessary to allow use of wide heat transfer plates with large ports of the type described above in general.
Moreover the clamp bolts must be dimensioned to resist the force required for the plate pack to be clamped sufficiently hard for a correct seal to be obtained. For each bolt not to be too thick and unwieldy to handle, a large number of bolts will be required in high pressure applications. In dimensioning for extremely high pressures, the problem sometimes arises that there is no space along the circumference of the plates for all the bolts that would be required.
Furthermore it is necessary to use strong frames, which makes the construction still more expensive. Especially in plate heat exchangers with a relatively small number of plates in which the frame cost constitutes a large part, this construction will be too expensive relative to the achieved heat transfer capacity.
In this context, also the type of plate heat exchanger as described in DE-A1-19716200 should be mentioned. This publication discloses a plate heat exchanger where all ports, i.e. also the ports for the different fluids, are positioned along one and the same line. The object stated in the DE publication is that it is desirable to obtain an improved distribution of the flow over the width of the heat transfer plates. The shape of the plate is essentially long narrow and rectangular, and the two ports for one of the fluids are positioned at the outer end of each short side of the plate whereas the two ports for the other fluid are positioned inside the same. As a result, the flow between the two outer ports is distributed along the whole width of the heat transfer plate, but the flow between the two inner ports will have a very poor distribution over the width of the plate. Thus, nor does this configuration offer a convenient solution to the above problems.
Mention should also be made of another type of plate heat exchanger where narrow plates are commonly used, viz. a very special type of heat exchanger, referred to as falling film evaporator. Such heat exchangers are described, for instance, in EP-A1-548360 and EP-A1-411123. A falling film evaporator is used to evaporate water or some other liquid from, for instance, fruit juice, sugar solutions or the like to obtain a higher concentration of the fruit juice or the sugar in the solution.
In such a falling film evaporator, a very special type of long narrow plates and a special sealing system are used. Vapour is in most cases supplied through a port which is positioned in the uppermost part and is passed downwards in every second plate interspace so as to be finally discharged from the evaporator through one or more ports positioned in the lowest part of the plate. The fluid from which liquid is to be evaporated is supplied through an upper port and discharged through a lower port. However, the upper port is not positioned in the upper part of the plate but it is displaced a considerable distance down towards the lower port. The liquid rises in a narrow, elongate preheating channel provided by gaskets from the inlet port until it reaches the upper part of the plate where it is then passed downwards on both sides of the preheating channel to an outlet port located in the lower portion of the plate. In EP-A1-411123, the inlet port is positioned in the lower portion of the plate, and in EP-A1-548360 the upper port is positioned just above the centre of the plate. This construction is only intended to be used for the very special flow conditions prevailing in falling film evaporators and would not function at all in conventional fields of application for ordinary plate exchangers. If this construction would be used for great flows, the pressure drop would be extremely high, which would cause an unsatisfactory degree of efficiency.
Moreover, the plate heat exchanger according to U.S. Pat. No. 4,708,199 should be commented on. This patent discloses a circular plate with a number of flanged through holes and plane openings which are alternatingly positioned on the same radius and with the same pitch in the circumferential direction. A number of plates are stacked one each other, each plate being rotated by a pitch in relation to the subjacent. The flanges round the holes provide a seal against the underside of the superposed plate and thus define this hole towards the flow area obtained between the plate in question and the plate above. Since the plates are rotated by a pitch relative to each other, every second port will communicate with every second flow area. This special construction has been developed for use in welded plate heat exchangers with the aim of not requiring two different plates that are alternatingly stacked on each other. However, this construction is not satisfactory when used at high pressures since circular plates give a maximum span in relation to a given heat transfer surface and are thus exposed to excessive loads.
There is thus no fully satisfactory conventional heat exchanger concept which can be used at high pressures. The variants that are available suffer from various drawbacks. For instance, they cause the construction of the frame to be unnecessarily heavy, the metal sheet to be poorly used or the flow to be unsatisfactorily distributed over the width of the plates. Above all, the latter problem of distribution must be solved since the efficiency of a plate heat exchanger is highly dependent on a good distribution of the flow of fluids over the whole width of the plate.